As shown in FIG. 1, the present invention relates to a cellular system having an orthogonal frequency division multiplexing (OFDM)-based forward link and a multi-carrier code division multiple access (MC-CDMA)-based forward link diversity method thereof.
A transmit diversity scheme is used for compensating degradation of transmission quality due to fading. In this scheme, a fading effect can be mitigated by transmitting the same signals by using more than two methods and combining the transmitted signals at a receiving unit. A CDMA system may employ a time diversity scheme, a frequency diversity scheme, a cell diversity scheme, a space diversity scheme, and a path diversity scheme.
The time diversity means to repeatedly transmit the same signal at different times with an interval and combine the signals at a receiving end, and the frequency diversity means to transmit a signal on multiple frequencies. The cell diversity scheme forms a multi-path by simultaneously establishing connections between more than two different base stations, located where a mobile station is currently located and in a different cell to which the mobile station will move, and the path diversity scheme receives multi-path signals, demodulates the received signals in the receiving unit, and combines the demodulated signals. In addition, the spatial diversity uses multiple receive antennas and combines signals received through the receive antennas. According to the spatial diversity, more than two receive antennas are provided in the base station because it is difficult to install two antennas in the mobile station.
According to the conventional transmit diversity method, a space time block coding (STBC) proposed by Siavash M. Alamouti in IEEE Journal on select areas in communication in 1998 is applied to the MC-CDMA. That is, data is transmitted by performing diversity encoding in space (antenna) and time domains.
In a basic mode where the number of transmit antennas is 1, assume that a modulation symbol transmitted at a time slot t is S0 and a modulation signal transmitted at a sequential time slot t+T is S1. In this assumption, the modulation symbol S0 is transmitted through the first antenna at the time slot t and a conjugate value (i.e., S0*) of the modulation symbol is S0 transmitted through the second antenna at the adjacent time slot t+T according to a transmit diversity scheme that adopts the Alamouti's STBC encoding scheme.
The second symbol S1 is transmitted through the second antenna at the time slot t and a negative conjugate value (i.e., −S1*) of the second symbol S1 is transmitted through the first antenna at the adjacent time slot t+T. That is, the Alamouti's STBC encoding scheme obtains a diversity gain by transmitting the respective modulation symbols through different antennas at two consecutive time slots.
The prior art that applies the STBC scheme to the MC-CDMA performs transmission by STBC-encoding two adjacent modulation symbols and multiplying the same code sequence for each antenna.
That is, as shown in FIG. 2, modulation symbols s2u(i),s2u+1(i) of the i-th mobile station are STBC-encoded and transmitted by using the same code sequence for each antenna. In more detail, the i-th modulation symbol transmitted through the first transmit antenna is s2u(i), and the i-th modulation symbol transmitted through the second antenna is (−s2u+1(i))* during the 2u-th OFDM symbol period. In addition, during the adjacent (2u+1)-th OFDM symbol period, the i-th modulation symbol transmitted through a first antenna is s2u+1(i) and the i-th modulation symbol transmitted through a second antenna is (s2u(i))*. As described, x* is a complex conjugate of x.
An encoded modulation symbol for the i-th mobile station is spread with a MC-CDMA orthogonal code c(i)=[c0(i)c1(i)c2(i), . . . , cN-1(i)]T allocated to the i-th mobile station, and combined with a signal of a different mobile station in each symbol period and transmitted through each antenna.
However, according to the conventional diversity technique, the STBC encoding is performed only between adjacent symbols among symbols transmitted to the same mobile station in the MC-CDMA system. Therefore, a diversity technique that is not restricted to symbols transmitted to the same mobile station is needed, and a variety of the diversity techniques also needs to be increased.
Particularly, it is difficult to apply the diversity technique to a 1 bit channel when the STBC encoding is performed between adjacent symbols.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.